The Costa Rica forward, who hasn't made an appearance for Arsenal this term after suffering an injury while on international duty, previously spent a period on loan with the La Liga outfit in the 2012/13 campaign.
He said: "I always wanted to come back because when I left I had a beautiful image of all the progress I made in 2012. Now I am very happy to come back.
"I will put all my talent at the disposal of the team. I will try to be an explosive, unbalanced and dangerous player. More than that, it is to contribute work, humility, sacrifice in the team. I hope everything goes well.
"The club has changed a lot. The stadium is spectacular. That gives me even more illusion than I had. I am happy and eager to be on the field with my teammates."
The move will be Campbell's sixth loan deal, having previously spent spells at the likes of Lorient, Olympiacos, Villarreal and Sporting Lisbon.
However, there are reports that this time there is no option for Real Betis to buy the 25-year-old.